U.S. patent number 4,351,697 [Application Number 06/336,980] was granted by the patent office on 1982-09-28 for printed wiring boards.
This patent grant is currently assigned to Western Electric Company, Inc.. Invention is credited to Daniel J. Shanefield, Fred W. Verdi.
United States Patent |
4,351,697 |
Shanefield , et al. |
September 28, 1982 |
Printed wiring boards
Abstract
A printed circuit board is made from a substrate having a copper
clad layer thereon by means of first providing spaced through-holes
through the substrate, mechanically scrubbing the surface of the
substrate and then sputter etching the surface of the substrate so
as to remove at least 50A. thereof from the surface and then vacuum
metallizing the through-holes subsequent to sputter etching without
breaking the vacuum and then finally forming a circuit pattern on
the substrate by usual subtractive techniques.
Inventors: |
Shanefield; Daniel J.
(Princeton Township, Mercer County, NJ), Verdi; Fred W.
(Lawrence Township, Mercer County, NJ) |
Assignee: |
Western Electric Company, Inc.
(New York, NY)
|
Family
ID: |
23318568 |
Appl.
No.: |
06/336,980 |
Filed: |
January 4, 1982 |
Current U.S.
Class: |
216/18; 134/1;
204/192.32; 205/125; 216/52; 29/852; 427/250; 427/307; 427/328;
427/97.2; 430/313; 430/314; 430/318 |
Current CPC
Class: |
H05K
3/4076 (20130101); Y10T 29/49165 (20150115) |
Current International
Class: |
H05K
3/40 (20060101); C23F 001/02 () |
Field of
Search: |
;156/643,645,656,659.1,661.1,666,901-902,630 ;174/68.5
;427/97,250,307,327,328 ;29/829,846,852 ;204/15,23,32R,192EC
;430/311,313,314,316,318 ;134/1,2,39,41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
J Vac. Sci. Technol., Vol. 11, No. 5, Sep./_Oct. 1974, A Sputtering
Technique for Coating the Inside Walls of Through-Holes in
Substrates by J. L. Vossen, pp. 875-877..
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Spivak; Joel F.
Claims
What is claimed is:
1. A method of making a printed circuit from a copper clad
substrate having a plurality of spaced through-holes therein
comprising:
(a) mechanically scrubbing the surface of the copper cladding;
(b) sputter etching the scrubbed surface so as to remove at least
50 A. of material therefrom;
(c) vacuum metallizing the through-holes subsequent to sputter
etching without breaking the vacuum created for sputter etching;
and then
(d) forming a circuit pattern on the substrate.
2. The method recited in claim 1 wherein mechanical scrubbing is
accomplished by bristle brush or compressed wheel scrubbing or
sanding.
3. The method recited in claim 2 using a 180-400 mesh abrasive.
4. The method recited in claims 1 or 2 wherein the step of vacuum
metallization is accomplished by sputtering.
5. The method recited in claims 1 or 2 wherein vacuum metallization
is by means of DC magnetron sputtering.
6. The method recited in claim 1 wherein the through-hole is
metallized with copper.
7. The method recited in claim 6 wherein said copper layer is at
least 4,000 A. thick.
8. The method recited in claim 1 wherein the time interval between
steps (a) and (b) is no more than three days.
9. A method of making a printed circuit from a copper clad
substrate having a plurality of spaced through-holes therein
comprising:
(a) mechanically scrubbing the surface of the copper cladding;
(b) sputter etching the scrubbed surface so as to remove at least
50 A. of copper therefrom;
(c) vacuum metallizing the through-holes without breaking the
vacuum created for sputter etching;
(d) applying photoresist over the copper surface;
(e) exposing the photoresist to ultraviolet light through a mask so
as to form a image therein;
(f) developing the image to expose a copper circuit pattern;
(g) electroplating copper over the exposed copper and
through-holes;
(h) forming a solder resist over the electroplated copper;
(i) removing the remaining photoresist; and
(j) etching away the copper which was under the photoresist removed
in step (i).
Description
TECHNICAL FIELD
This invention relates to the manufacture of printed wiring boards
and, more particularly, to the vacuum metallization of
through-holes in the manufacture of printed wiring boards.
BACKGROUND OF THE INVENTION
Printed wiring boards are in common use in the electronics and
telecommunications and other industries for creating conductive
paths on interconnector devices. These conductive paths may be
formed on one or both sides of a nonconductive substrate or one
side of multiple substrates laminated together. It is generally
required when employing double sided or multilayered printed wiring
boards that through-holes be provided so as to interconnect
conductive paths and interconnect devices between sides of the
printed wiring boards. It is common to use as a starting material
for the printed wiring board, a substrate having a blanket metal
coating formed or laminated thereover and then providing the
through-holes by drilling through the substrate in predetermined
areas and subsequently plating the holes formed by drilling.
Currently, metallization of these through-holes is accomplished by
wet chemical processing including electroless plating techniques.
Since the cost of vacuum metallization has been getting lower as
large scale vacuum equipment becomes more available and reliable,
while the costs of the alternative chemicals involved in wet
chemical processing continue to rise, it would be desirable to
replace the wet chemical techniques used for through-hole plating
with more economical and more environmentally desirable dry
metallization techniques such as vacuum metallization. Still
another reason for reducing chemical processing wherever possible
is the reduction of toxic chemical waste generated in chemical
processing techniques. While it is not possible to totally
eliminate wet chemical tecniques when using what is known in the
art as subtractive technology for forming printed circuit boards
employing metal laminated panels as a starting material, at least
the steps of electroless deposition can be eliminated by
substituting vacuum metallization for the electroless metallization
of through-holes. While the purpose of the metallization is to
cover the through-holes with a conductive metal deposit, it is
necessary to overlap the through-hole metallization with the metal
cladding on the surface of the board. Furthermore, the vacuum
deposited layer must adhere well to both the through-hole and the
metal cladding to prevent delamination. In attempting to make the
substitution from electroless plating to vacuum metallization,
however, it was discovered that the vacuum deposited copper had
insufficient adhesion to the originally laminated copper foil such
that the final circuits often exhibited layer-plating or
interfacial delamination failure of the bond between the vacuum
metallized layer and the original laminate or between a later
electroplated copper layer on the conductor paths of the circuits
and the original laminated copper. Industry standards for adhesion
require at least a five pound per inch peel strength with a
90.degree. peel and preferably one seeks at least an 8 pound per
inch peel strength. Until the discovery of the cause of the lack of
adhesion and the removal or solution of this cause, vacuum
metallization of the through-holes could not be accomplished for
practical commercial use.
SUMMARY OF THE INVENTION
A method for manufacturing printed circuit boards employing a
copper clad substrate having a plurality of spaced through-holes
therein includes the steps of (a) mechanically scrubbing the
surface of the substrate, (b) sputter etching the surface of the
substrate so as to remove at least 50 A. from said surface (c)
vacuum metallizing the through-holes subsequent to sputter etching
without breaking the vacuum, and (d) forming a circuit pattern on
the substrate.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is an elevational schematic view of a vacuum sputtering
apparatus useful in practicing the invention.
DETAILED DESCRIPTION
As previously indicated, prior attempts to vacuum metallize
through-holes in copper clad printed circuit boards resulted in
circuit boards which exhibited delamination due to poor adhesion
between the vacuum deposited copper and the underlying copper
cladding on the surface of the printed circuit board. It was
initially believed that this delamination problem could be solved
by simply cleaning the surface of the copper prior to vacuum
metallization. Attempts were made to clean the copper surface by
either mechanically scrubbing the surface, chemically cleaning the
surface or sputter etching the surface prior to vacuum
metallization. However, adhesion problems still persisted and
delamination failure still occured. We have discovered what we
believe to be the cause of the insufficient adhesion as well as a
solution to the problem. It is believed that the insufficient
adhesion is caused by a chromium oxide layer formed on the surface
of the copper clad printed circuit boards. It is believed that a
thin chromium layer is placed over bare copper clad printed circuit
boards by manufacturers in order to protect the surface which
oxidizes on exposure to air to form chromium oxide. We then
discovered that by combining mechanical scrubbing of the surface of
the substrate followed by sputter etching of the surface of the
substrate so as to remove at least 50 A. of the surface, the
surface is sufficiently precleaned such that a vacuum metallized
layer formed after the precleaning and without exposure to air has
sufficient adhesion (at least eight pounds per inch peel strength)
such that circuits formed on the substrate do not later delaminate.
It is hypothesized that mechanically scrubbing the surface depletes
the surface of most of the chromium oxide while sputter etching
reduces the remaining chromium oxide to metallic chromium which
does not act to inhibit adhesion.
Based upon the aforementioned discovery, a printed circuit board
may be manufactured in accordance with our invention employing
commercially available copper clad substrates in accordance with
the following procedures. The substrates may either have a
plurality of spaced through-holes already provided therein or one
may form such spaced through-holes by any known techniques. The
method comprises: (a) mechanically scrubbing the surface of the
substrate; (b) sputter etching the surface of the substrate so as
to remove at least 50 A. from said surface; (c) vacuum metallizing
the substrate surface and through-holes subsequent to sputter
etching and prior to any exposure to oxygen; and (d) forming a
circuit pattern on the substrate by any of the known subtractive
techniques.
It should be understood that the term "vacuum metallization" or
"vacuum deposition" as used herein includes any of the known vacuum
techniques for depositing a metallic layer including vacuum
evaporation, sputtering, ion plating and electron beam vacuum
deposition.
The general technique for forming printed circuit patterns by means
of subtractive processing is very well known in the art. The
technique generally involves processing the copper laminated
printed circuit board having plated through-holes therein by
applying a resist pattern over the copper surface so as to
delineate the desired circuit pattern followed by etching of the
exposed copper surface. Hence, when the resist is removed, the
copper plated circuitry and the surface of the holes which are
copper plated remain. Examples of the subtractive process can be
found with reference to U.S. Pat. Nos. 3,673,680 and 4,135,988 both
of which are incorporated herein by reference. The latter patent
also teaches methods of forming apertures or through-holes through
copper clad printed circuit boards.
In accordance with the present method, laminated panels of
fiberglass reinforced epoxy, for example, laminated with copper
foil on its surface would have holes drilled therethrough and then
the surfaces would be deburred and mechanically scrubbed. The
laminate would then be placed in a vacuum apparatus where the
copper surface would be further treated by sputter etching so as to
remove at least 50 A. from the surface thereby also reducing any
oxide of chromium or other weak oxide on the surface. Without
breaking the vacuum, a thin film of copper is then vacuum deposited
in the through-holes as well as on the major surfaces. Photoresist,
thick copper electroplating, tin alloy electroplating, photoresist
stripping, etching and other steps as are well known in the art
would then be carried out as usual so as to form the desired
circuit pattern. In other words, except for the precleaning and
vacuum metallization steps which replace the usual electroless
metallization of the through-holes, standard subtractive processing
is used to complete the manufacture of the circuit boards.
Typical parameters for preparing printed circuit boards in
accordance with the invention are as follows:
(a) the scrubbing step may be performed utilizing a bristle brush
scrubber having abrasive powder embedded in the bristles, such as a
commercially available scrubber manufactured by the Chemcut Company
of State College, Pa. This scrubber is used water wet.
Alternatively, one may use a sanding machine such as a Somaca
Company, Chicago, Ill., sander to wet sand or scrub the substrate
surface. Suitable grit sizes for the abrasive powder of the bristle
brush or the sander are from 180 to 400 mesh. It should be
understood that other scrubbing techniques may also be suitable and
that other scrubbing techniques may also be suitable and that other
mesh or grit sizes can also be used as well, and that this
invention is not limited by any particular scrubbing technique or
mesh size of abrasive material used for scrubbing;
(b) the step of sputter etching can be carried out, for example,
under RF or DC conditions. Typically, for RF sputter etching one
may use a wattage of approximately 500 watts at 13 to 14 MHZ and an
argon pressure of from 5 to 100 microns, approximately 10 microns
being preferred. Usual etch time, in order to etch at least 50 A.
from the surface, is approximately 3 minutes under these
conditions. Alternatively, one may utilize a magnetron DC sputter
etch employing voltages of, for example, 250 to 350 volts with
powers of about 2 to 3 watts per square inch of substrate material
and a vacuum of from 5 to 100 microns of argon; and
(c) for copper sputter deposition, it has been found to be
preferred to use argon pressures of from 0.5 to 100 microns and
powers of about 45 to 100 watts per square inch of cathode.
In order to insure sputter deposited layers which can be copper
plated free from through-hole voids, it has been found that the
thickness of the copper deposit should be at least 4,000 A. and
preferably 8,000 A. Still another discovery is that the time
interval between scrubbing and etching can affect adhesion. In
order to insure the required adhesion of the sputtered copper
deposit, the time interval should preferably not exceed 3 days
between scrubbing and etching of any printed circuit board.
EXAMPLE
A commercial printed circuit board substrate having a copper
cladding laminated to at least one side thereof is drilled so as to
incorporate through-holes therein in accordance with a desired
circuit pattern to be provided. The board is deburred subsequent to
drilling and then wet scrubbed utilizing a Chemcut Company bristle
brush scrubber. The scrubbed surface is then rinsed and dried and
placed in a DC magnetron sputtering apparatus 10 such as is
depicted in the FIGURE. The apparatus 10 comprises a copper coated,
stainless steel walled vacuum chamber 12 which is provided with a
gas inlet port 14 and a vacuum port 16. Within the chamber 12 and
insulated from its walls is a slidable, copper coated, stainless
substrate carrier plate 18 which supports a spaced substrate 20 by
means of conductive spacers 22. An insulating handle 24 extends
from the carrier 18 through the chamber wall 30 in a manner so as
to maintain the vacuum. The handle moves the substrate from its
first position, as shown, to a second position under a copper
cathode 26. During sputter etching the substrate and carrier are in
the first position and act as the cathode by being negatively
charged via contact 28. During sputter deposition the substrate 20
is slid into the second position under the copper cathode 26 and is
itself floating electrically. The chamber wall 30 acts as the anode
for both etching and deposition. An insulator 32 is provided over
the copper cathode 26 adjacent the chamber wall 30. It has been
found by a co-worker that more efficient sputtering into the
through-holes is obtained if the substrate 20 is raised from the
sputtering carrier plate 18 by by a small amount, such as about
3/16 of an inch. The sputtering apparatus 10 is evacuated by means
of a vacuum pump to a vacuum of approximately 10.sup.-6 Torr and
argon is bled back into the apparatus through the argon inlet 14 to
a pressure of approximately 10.sup.-2 Torr. While the substrate and
carrier are in the position shown, a plasma is created by applying
approximately 300 volts DC across the anode 30 and substrate
cathode 20. Sputter etching is continued for approximately five
minutes and thence the sputtering apparatus 10 is again evacuated
to approximately 5.times.10.sup.-3 Torr and backfilled with argon
to approximately 5 microns without breaking the vacuum of the
apparatus 10. This prevents oxidation of any remaining chromium on
the surface of the copper clad substrate 20. An 8,000 A. thick
copper layer is then sputter deposited at a voltage of
approximately 400 to 500 volts DC in the apparatus 10 by sliding
the substrate 20 from its first position where etching takes place
to its second position under the copper cathode 26. This copper
deposit coats the through-hole and copper clad surfaces of the
substrate. After copper sputter deposition, the substrate is
removed from the sputtering chamber and a photoresist such as
Dupont Riston 1215 photoresist is applied over the surface. The
photoresist is then imaged utilizing a mercury vapor, ultraviolet
lamp through a photographic negative mask. The imaged photoresist
is then developed with trichloroethane so as to remove the
unexposed portions of the photoresist. Copper is then electroplated
to a thickness of about 1.5 mils over those areas from which the
photoresist has been removed and in the through-holes. Thereafter,
a solder etch-resist is electroplated over the same areas to a
thickness of approximately 500 microinches. The remaining
photoresist is then stripped with methylene chloride and the copper
underlying the stripped photoresist is then removed by etching with
a solution of ferric chloride or an ammoniacal cupric chloride
solution and then rinsed thereby producing the final printed
circuit board which is ready for the attachment of discrete devices
thereon.
It is to be understood that the above-described embodiments are
simply illustrative of the principles of the invention. Various
other modifications and changes may be devised by those skilled in
the art which will embody the principles of the invention and fall
within the spirit and scope thereof.
* * * * *